This invention relates to high-strength composite fasteners and is particularly directed to fiber-reinforced thermoplastic fasteners, such as rivets, and to a method for fastening structural components, especially composite components, with such fasteners, utilizing novel tooling for softening and reshaping the thermoplastic material of the fastener to achieve fastening therewith.
Composite materials in the form of fiber-reinforced plastics or resins are replacing metal structural components in the aerospace industry because of their high strength to weight ratios and high stiffness. The fibers provide strength and stiffness, and the matrix provides the binding material for these fibers. One form of organic composites presently employed is of the thermosetting type which is fabricated to a predetermined shape and then cured at a specific elevated temperature.
A new generation of composites are of the thermoplastic type. In these composites, the matrix softens when brought up to a specific elevated temperature and solidifies on cooling. This property provides the opportunity to shape and reshape the thermoplastic by merely bringing the structural component up to or above this forming temperature.
While both types of composite materials, thermosetting and thermoplastic, have gained wide usage in aerospace structures, their drawback has consisted in a lack of a suitable method for attachment between the structural composite components. The bonding together of composite structures is not sufficiently dependable and separation, that is, peeling of the bonding, often occurring. Presently, the bonding of composite components is supplemented by limited fastening.
The more conventional method for fastening composites is by the use of metal fasteners. However, aluminum fasteners are generally not compatible with fiber-reinforced composites, and while titanium fasteners are generally compatible with such composites, such fasteners are heavy and expensive. Further, such fasteners require large heads to avoid "pulling through" failure.
U.S. Pat. No. 2,510,693 to Green discloses a rivet formed of plastic material having fibers therein running longitudinally in the stem of the rivet and extending into the head of the rivet in laterally spreading relation. In fastening a pair of components by means of the rivet, the end of the rivet is upset by means of pressure applied thereto by a heated riveting tool to form a head. However, the fibers are relatively randomly distributed in the stem and in the head of the rivet, and the resulting rivet lacks high-tension capability.
U.S. Pat. No. 2,685,813 to Lampman, et al, discloses a prefabricated glass fiber rivet body comprised of a plurality of glass fiber threads grouped together to extend longitudinally in the rivet body, the body being impregnated with a partially cured plastic binder, and a thermoplastic sleeve, the rivet body being adapted to be upset by heat and pressure to provide a solid-headed rivet. In use, the rivet body is subjected to heat and pressure to laterally crush outwardly the ends of the glass fiber threads to form the heads in countersunk portions of the bores of the components to be fastened together. In this arrangement, wherein the ends of the rivet body are subjected to heat and pressure to laterally crush the fibers outwardly, there is a lack of control of the orientation of the fibers in the rivet, particularly in the heads of the rivet, and the resulting rivet has only low-tension capability. Further, rivets of this type can only be used with countersunk holes in the components being fastened and cannot be employed for the production of protruding head fasteners or rivets, wherein countersunk holes are not provided in the structural components to be fastened.
U.S. Pat. No. 4,478,544 to Strand discloses a reinforced thermoplastic rivet comprising a plurality of substantially parallel continuous longitudinally extending carbon fibers encapsulated in a B-stage thermoset resin and a plurality of longitudinally extending polyamide fibers enveloping the carbon fibers. The rivet is deformable to form a rivet head upon heating. However, in this procedure the fibers in the head are crushed, without control of the position and direction of the fibers.
Due to the haphazard non-uniform orientation of the fibers in the plastic matrix of the stem and/or head of the rivets disclosed in each of the above patents, while such rivets exhibit satisfactory shear capability, they lack good capability in tension.
It is an object of the present invention to provide novel high-strength composite fasteners, particularly rivets, having superior high-shear and high-tension capability.
Another object of the invention is the provision of an improved high-strength composite fastener, particularly rivets, especially designed for aircraft applications, and wherein suitable reinforcing fibers are embedded in a high temperature thermoplastic matrix and wherein the fibers are so oriented in both the shank and heads of the fastener so as to result in a fastener having both high-shear and high-tension performance.
A further object of the invention is to provide a high-strength composite fastener, e.g., rivets, of the above type, and to a novel method and tooling for upsetting such fastener to form a second head and produce fastening of a plurality of structural components by such fastener.